There are a lot of great resources on the interwebs for OGL game masters to use and make their jobs running games much easier.

• 5e Mechanics for Cailore Fantasy: Every DM does some things differently, these are rules I use when DMing 5e.
  
• Lore Masters’ Guild: Access to what’s behind the screen in Cailore Fantasy.
• Gnome Stew: The Gaming Blog. In a multiverse of flakey GM publishing drivel on the interwebs, Gnome Stew offers something that is very professional, with extra chucks of Gnome.
• Mad Irishman Character Sheets: These are some of the very best character sheets to be found on the interwebs.
• Encounter Level Calculator: Need to know how much of a threat that OGL encounter is? This calculator will give you some good estimations of how tough an encounter is.
• Cartographers’ Guild: A forum specializing in map making for fantasy and fiction. Game Masters can find a treasure trove of beautiful maps here to help them on the way.
• DriveThru RPG: A great place to download inexpensive .pdfs of old-school role playing games and supplements.
• Cailore Rules Document: Get the free, online system rules used by Cailore Fantasy.
• Fantasy Shields: You can use this online tool to create good-looking coats of arms for your campaign.
• Donjon: Random generators for role playing games, since at least 2009.
• Pathfinder Settlement Creator: Uses OGL rules to generate a nice stat-block for your settlement.
• Fantasy Grounds Developer Notes: Run your game with friends on the interwebs.
• Using Fantasy Grounds with Syrinescape: Because nothing should be easy.
• Monster Advancer: Creature your own monsters based on OGL templates, give superpowers to that Moose, make an infernal-vampire badger!
• The D&D Wiki: Information from the SRD plus tons of 3rd party rules and material.
• Joe Sweeney’s Kick-Ass Mapping Tutorials: Seriously, this guy is the Bob Ross of Campaign Cartographer.
• Fantasy Name Generator: Stuck on a name? Emily has compiled an excellent series of name generators to help you get your name on.
• Dynamic Magic Item Creation: Pathfinder has a fun and interesting way to spice up magic item creation for OGL games.
• Fantastic Maps: Great maps and tutorials on making your own maps by Jonathan Roberts.
• Cool Artistic Character Sheets.
• NaturalCrit & Home Brewery: A great tool for formatting your homebrew to look legit as a 20 HD kobold and as a bonus,Jérôme Poslednik has provided a D&D Achievement Badge maker to immortalize your players’ achievements.
• Export Your Fantasy Grounds .XML to a PDF Character Sheet: For as much as I love Fantasy Grounds, their decision to go without a character sheet export still pisses me off.
• Do the Math for Me to Resize my Map File for VTT: OK, I hate when people say math is hard, but sometimes everyone needs a fast and easy way to help out. This does that.
• CCA Master Mapping Tile Library by StoryWeaver Games presents a trove of material tiles you can use on your own game maps. While you are there, check around the site, Joe Sweeney and friends produce some great RPG stuff.